The lack of antigen-specific cellular responsiveness seen in patients with chronic schistosomiasis has been found to be associated with inhibitory factors, both in serum and in mononuclear cell populations of cases with the chronic infection. Another type of immune modulation, involving serum IgM and IgG antibodies to a proteoglycan antigen of the schistosome, has been found in patients with acute or chronic schistosomiasis. The nature of immune complexes in patients with acute schistosomiasis is under way. Specificity of the IgE response to various filarial antigens, as assessed by histamine release, is being studied in collaboration with workers in Madras, India. This, as well as their cellular immune responsiveness, is of particular interest in tropical pulmonary eosinophilia, as well as in other clinical syndromes of filariasis.